With recent development in technology for miniaturizing semiconductors and accelerating the speed of the semiconductors, the amount of data communicated between devices or LSIs (Large Scale Integrations) provided in the devices is increasing more than ever. On the other hand, a strict restriction is posed on the number of terminals (pads) in an LSI, which affects the packaging cost. Accordingly, for the purpose of achieving high-speed data communication with a fewer number of terminals in an LSI, the interface standards employing serial communication have widely prevailed.
In general, a bus connection is difficult in a serial line. One example of network topologies used to connect a plurality of communication devices is a ring topology. With the ring topology, a communication device that serves as a relay station connected between a source communication device, from which data is transmitted, and a destination communication device, to which the data is transmitted, needs to be active at any time to perform relay processing. For example, Patent Literature 1 discloses a conventional technique that, in order to reduce the amount of power consumed by such a communication device serving as the relay station, causes the relay station to switch to bypass mode. In this bypass mode, a protocol processing unit that operates in a logical layer, which is unnecessary for the relay processing, is made inactive.